Node is a region in atomic orbital where there is no possibility of finding electrons. That is no electron can be found there (region of $\psi^2 = 0$) Take example of say $\mathrm{2s}$ orbital: as electrons cannot be found in node, then how can it spend time in first lobe (spherical) and in second lobe (spherical) separated by nodal region where it cannot go?
And because of course it is electron and its possibility to be found that marks up possible boundary of orbital.